Apparatus for driving liquid crystal display panel for different size images

Abstract: In an apparatus for driving a liquid crystal display panel (1) having signal lines (SG₀,SG₁, ... ) and scan lines (SL₀,SL₁,...), the operation of a signal line driving circuit (2-1,2-2) for sequentially driving the signal lines is started by a horizontal start timing signal (HS) which is obtained by delaying a horizontal synchronization signal (HSYNC) with a first time period (HSET). Also, the operation of a scan line driving circuit (3) for sequentially driving the scan lines is started by a vertical start timing signal (VS) which is obtained by delaying a vertical synchronization signal (VSYNC) with a second time period (VSET). The first time period and the second time period are changed in accordance with a size of one frame of a video signal. Use: Multi-synchronization LCD panel. Display images having different sizes at a center portion of the panel.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a liquid crystal display (LCD) system, and more particularly, to an apparatus for driving a multi-synchronization type LCD panel for different size images. Description of the Related Art There has been known a multi-synchronization type deflecting apparatus for a cathode-ray tube (CRT) panel which can properly display images having different numbers of scan lines at a center portion of the panel. On the other hand, since LCD panels are thinner in size and lower in power consumption with a lower power supply voltage as compared with CRT panels, the LCD panels have recently been applied to personal computers, word processors, color telereceivers, and the like. However, the multi-synchronization type deflecting system of the CRT panels cannot be applied to the multi-synchronization type driving system of the LCD panels, due to the difference in driving (deflecting) methods therebetween. In a prior art apparatus for driving an LCD panel having M signal lines, N scan lines and M × N liquid crystal cells connected to the signal lines and the scan lines, the operation of a signal line driving circuit for sequentially driving the signal lines is started by a horizontal start timing signal which is obtained by delaying a horizontal synchronization signal with a constant time period. Similarly, the operation of a scan line driving circuit for sequentially driving the scan lines is started by a vertical start timing signal which is obtained by delaying a vertical synchronization signal with a constant time period. As a result, an image having a size different from M × N dots is ill-balanced at an upper portion or a left portion of the LCD panel. This will be explained later in detail. Patent Abstracts of Japan, vol. 013, No. 495 (P-956), Nov. 1989, discloses an apparatus to make a display at an optional position even with optional display capacity without increasing a display transfer speed by providing a column or row driving means with an offset means which can set a display start position optionally. When an offset means is provided with the column driving means, input display data which are transferred are fetched in order from the position corresponding to the set value of the initial-stage latch in the column driving means and column electrodes are driven according to the fetched data. Further, when an offset means is provided to a row driving means, row electrodes are driven in order from the electrode corresponding to the set value. When the capacity of the display data is smaller than the display capacity of the dot matrix display device, the display data is displayed on the center of the display panel of the dot matrix display device by selecting the set value properly. Thereby the data can be displayed at an optional position without using the transfer speed of the display data. Patent Abstracts of Japan, vol. 015, No. 360 (P-1250), Sept. 1991, discloses a means for changing the position of an image displayed on a liquid chrystal panel of an image display device which projects the image displayed on plural liquid chrystal panels to a screen in the horizontal and vertical directions on a display panel. For that purpose a gate timing signal and a source timing signal are inputted in a gate driver and a source driver by changing the positions of the signals by means of a gate timing signal movement and a source timing signal movement circuit. Since scanning start timing in the vertical and horizontal directions it moves in the liquid crystal panel, the displayed image can be moved in the vertical and horizontal directions. Thus, the convergence adjustment of the projection type image display device is facilitated and the adjusting stage is simplified. WO-A-90/12367 discloses a method for controlling the vertical centering of sequential frames of a raster image on a raster imaging surface. It involves detecting the vertical location of the last line of the raster image in the frame preceding the frame to be controlled; and controlling the beginning of the raster image of the controlled frame on the basis of the location of the detected last line. The method further provides for automatic centering of the image on the imaging surface and for compression of the raster image. SUMMARY OF THE INVENTION It is an object of the present invention to provide a multi-synchronization type driving apparatus for an LCD panel which can display an image having different size images at a center portion thereof. This object is met by an apparatus as defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more clearly understood from the description as set forth below, in comparison with the prior art, with reference to the accompanying drawings, wherein: Fig. 1 is a block circuit diagram illustrating a prior art apparatus for driving an LCD panel; Fig. 2 is a detailed block circuit diagram of the signal line driving circuit of Fig. 1; Fig. 3 is a detailed circuit diagram of the horizontal timing generating circuit of Fig. 1; Figs. 4A through 4F are timing diagrams showing the operation of the circuits of Figs. 2 and 3; Figs. 5A and 5B are timing diagrams for explaining an image signal of one horizontal line. Fig. 6 is a detailed block circuit diagram of the scan line driving circuit of Fig. 1; Fig. 7 is a detailed circuit diagram of the vertical timing generating circuit of Fig. 1; Figs. 8A through 8F are timing diagrams showing the operation of the circuits of Figs. 6 and 7; Figs. 9A through 9D are timing diagrams for explaining a video signal of one frame in the prior art; Fig. 9E is a diagram showing an image displayed on the LCD panel of Fig. 1; Fig. 10 is a detailed circuit diagram of the timing generating circuit of Fig. 1; Fig. 11 is a block circuit diagram illustrating a first embodiment of the apparatus for driving an LCD panel according to the present invention; Fig. 12 is a detailed block circuit diagram of the image size determining circuit of Fig. 11; Fig. 13 is a diagram showing the content of the look-up table of Fig. 12; Fig. 14A through 14D are timing diagrams for explaining a video signal of one frame according to the present invention; Fig. 14E is a diagram showing an image displayed on the LCD panel of Fig. 11; Fig. 15 is a block circuit diagram illustrating a second embodiment of the apparatus for driving an LCD panel according to the present invention; Fig. 16 is a detailed circuit diagram of the image size determining circuit of Fig. 15; Fig. 17 is a diagram showing the content of the look-up table of Fig. 15; Fig. 18 is a detailed circuit diagram of the signal processing circuit of Fig. 15; Fig. 19 is a circuit diagram of the data compression circuit of Fig. 18; Figs. 20A, 20B and 20C are timing diagrams showing the operation of the circuit of Fig. 19; Fig. 21 is a circuit diagram of the layer expansion circuit of Fig. 18; and Figs. 22A through 22H are timing diagrams showing the operation of the circuit of Fig. 21. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before the description of the preferred embodiments, a prior art apparatus for driving an LCD panel will be explained with reference to Figs. 1, 2, 3, 4A through 4F, 5A, 5B, 6, 7, 8A through 8F, and 9A through 9E. In Fig. 1, which illustrates a prior art apparatus for driving an LCD panel, reference numeral 1 designates an LCD panel having M × N dots where M=1280 and N=1024. That is, the LCD panel 1 has signal lines SG i (i=0, 1, ... , 1279) driven by signal line driving circuits 2-1 and 2-2, 1024 scan lines SL j (j=0, 1, ... , 1023) driven by a scan line driving circuit 3, and pixels each connected to one of the scan lines and one of the signal lines. Also, each of the pixels is formed by a thin film transistor (TFT) Q ij and a liquid crystal cell C ij . An analog/digital (A/D) converter 4 receives color signals R, G and B of a video signal and performs an A/D conversion thereupon by using a clock signal ACK. The digital output of the A/D converter 4 is supplied to a signal processing circuit 5 clocked by a clock signal TCK. The signal processing circuit 5 includes a layer expansion circuit which will be later explained. Two outputs DA and DB of the signal processing circuit 5 are supplied to two D/A converters 6-1 and 6-2, respectively, clocked by a clock signal DCK, and the converters are also connected to the signal line driving circuits 2-1 and 2-2, respectively. A timing generating circuit 7, which includes a phase-locked loop (PLL) circuit, receives a horizontal synchronization signal HSYNC to thereby generate various clock signals HCK, VCK, CK in addition to the clock signals ACK, TCK and DCK. In this case, the clock signal HCK is used for sequentially driving the signal line driving circuits 2-1 and 2-2, and the clock signal VCK is used for sequentially driving the scan line driving circuit 3. A horizontal timing generating circuit 8 generates a horizontal start timing signal HS and transmits it to the signal line driving circuits 2-1 and 2-2, to thereby start the operation thereof. Also, a vertical timing generating circuit 9 generates a vertical start timing signal VS and transmits it to the scan line driving circuit 3, to thereby start the operation thereof. In Fig. 2, which is a detailed block circuit diagram of the signal line driving circuit 2-1 (2-2) of Fig. 1, shift registers (D flip-flops) 21-0, 21-1, ... , 21-639 are serially-connected. The horizontal start timing signal HS is shifted through the shift registers 21-0, 21-1, ... , 21-639 by the clock signal HCK. The outputs of the shift registers 21-0, 21-1, ... , 21-639 control switching circuits 22-0, 22-1, ... , 22-639, respectively, which receive the data signal of the D/A converter 6-1 (6-2). Thus, the switching circuits 22-0, 22-1, ... , 22-639 sequentially drive the signal lines SG 0 , SG 1 , ... , SG 639 , respectively, in accordance with the video signal. In Fig. 3, which is a detailed circuit diagram of the horizontal start timing circuit 8 of Fig. 1, this circuit 8 includes a counter 801 which counts the clock signal CK and is cleared by the horizontal synchronization signal HSYNC. Also, an exclusive OR circuit 802 and an inverter 803 compare the content of the counter 801 with a constant value HSET. That is, only when the content of the counter 801 coincides with the constant value HSET, is the output of the inverter 803, i.e., the horizontal start timing signal HS, "1". The operation of the horizontal start timing circuit 8 and the signal line driving circuit 2-1 (2-2) will be explained with reference to Figs. 4A through 4F. That is, the horizontal start timing generating circuit 8 counts the clock signal CK after the horizontal synchronization signal HSYNC is made "0" (low) as shown in Fig. 4A. As a result, after the constant time period HSET has passed, the horizontal timing generating circuit 8 generates the horizontal start timing signal HS as shown in Fig. 4B. Therefore, the horizontal start timing signal HS is supplied to the first stage of the shift registers, i.e., the shift register 21-0, and the horizontal start timing signal HS is shifted through the shift registers 21-0, 21-1, ... , 21-639 by the clock signal HCK as shown in Fig. 4C. As a result, the signal lines SG 0 , SG 1 , ... , SG 639 are sequentially driven by the AND logic between the output signals of the shift registers 21-0, 21-1, ... , 21-639 and the data of the video signal as shown in Figs. 4D, 4E and 4F. Thus, in the prior art, the relationship between a horizontal line of the video signal as shown in Fig. 5A and the horizontal start timing signal HS as shown in Fig. 5B is constant. In Fig. 6, which is a detailed block circuit diagram of the scan line driving circuit 3 of Fig. 1, shift registers (D flip-flops) 31-0, 31-1, ... , 31-1023 are serially-connected for driving the scan lines SL 0 , SL 1 , ... , SL 1023 , respectively. The vertical start timing signal VS is shifted through the shift registers 31-0, 31-1, ... , 31-1023 by the clock signal VCK. In Fig. 7, which is a detailed circuit diagram of the vertical start timing circuit 9 of Fig. 1, this circuit 9 includes a counter 901 which counts the horizontal synchronization signal HSYNC and is cleared by a vertical synchronization signal VSYNC. Also, an exclusive OR circuit 902 and an inverter 903 compare the content of the counter 901 with a constant value VSET. That is, only when the content of the counter 901 coincides with the constant value VSET, is the output of the inverter 903, i.e., the vertical start timing signal VS, "1". The operation of the vertical start timing circuit 9 and the scan line driving circuit 3 will be explained with reference to Figs. 8A through 8F. The vertical start timing generating circuit 9 counts the horizontal start signal HS after the vertical synchronization signal VSYNC is made "0" (low) as shown in Fig. 8A. As a result, after the constant time period VSET has passed, the vertical timing generating circuit 9 generates the vertical start timing signal VS as shown in Fig. 8B. Therefore, the vertical start timing signal VS is supplied to the first stage of the shift registers, i.e., the shift register 31-0, and the vertical start timing signal VS is shifted through the shift registers 31-0, 31-1, ... , 31-1023 by the clock signal VCK as shown in Fig. 8C. As a result, the scan lines SL 0 , SL 1 , ... , SL 1023 are sequentially driven by the output signals of the shift registers 31-0, 31-1, ... , 31-1023 as shown in Figs. 8D, 8E and 8F. Thus, in the prior art, even if an image having 1152 × 900 dots as shown in Fig. 9B, that is smaller than an image having 1280 × 1024 dots as shown in Fig. 9A, is displayed on the LCD panel 1 having 1280 × 1024 dots, the timing of the horizontal start timing signal HS is constant as shown in Fig. 9C, and the timing of the vertical start timing signal VS is constant as shown in Fig. 9D. As a result, as shown in Fig. 9E, a 1152 × 900 dot image is ill-balanced at an upper-left portion of the LCD panel 1. In Fig. 10, which is a detailed circuit diagram of the timing generating circuit 7 of Fig. 1, this circuit 7 includes a PLL circuit formed by a phase comparator 701, a low-pass filter 702, a voltage controlled oscillator 703, and a 1/n frequency divider 704. That is, the phase comparator 701 compares the phase of the horizontal synchronization signal HSYNC with that of the output signal of the 1/n frequency divider 704. The low-pass filter 702 includes a capacitor which is charged or discharged by the output of the phase comparator 701. The voltage controlled oscillator 703 generates the clock signal CK whose frequency corresponds to the output of the low-pass filter 702. Thus, the frequency of the clock signal CK is maintained at a definite value determined by the horizontal synchronization signal HSYNC. Note the value n of the frequency divider 704 corresponds to the sampling number of the clock signal CK for one horizontal line of the video signal. The timing generating circuit 7 also includes a frequency divider circuit formed by a plurality of frequency dividers which generate the clock signals ACK, TCK, DCK, HCK and VCK. In Fig. 11, which illustrates a first embodiment of the present invention, the value HSET of the horizontal timing circuit 8 and the value VSET of the vertical timing circuit 9 are variable, and the values HSET and VSET are changed by an image size determining circuit 10 which calculates an image size of one frame of the video signal in accordance with the horizontal synchronization signal HSYNC and the vertical synchronization signal VSYNC. The image size determining circuit 10 is formed by a circuit as illustrated in Fig. 12. In Fig. 12, reference numeral 1001 designates a frequency-to-voltage converter for receiving the horizontal synchronization signal HSYNC to generate a voltage V H in response to the frequency of the horizontal synchronization signal HSYNC. Also, reference numeral 1002 designates a frequency-to-voltage converter for receiving the vertical synchronization signal VSYNC to generate a voltage V v in response to the frequency of the vertical synchronization signal VSYNC. The voltages V H and V v are converted by analog-to-digital converters 1003 and 1004 into digital values f H and f v , respectively. Then, the digital values f H and f v are supplied to a look-up table 1005, which in turn generates the values HSET and VSET. The look-up table 1005 is formed by a random access memory (RAM) or a read-only memory (ROM) for storing the values HSET and VSET as shown in Fig. 13. Thus, according to the first embodiment, even if an image having 1152 × 900 dots as shown in Fig. 14B, that is smaller than an image having 1280 × 1024 dots as shown in Fig. 14A, is displayed on the LCD panel 1 having 1280 × 1024 dots, the timing of the horizontal start timing signal HS is variable as shown in Fig. 14C, and the timing of the vertical start timing signal VS is variable as shown in Fig. 14D. As a result, as shown in Fig. 14E, a 1152 × 900 dot image is balanced at a center portion of the LCD panel 1. In Fig. 15, which illustrates a second embodiment of the present invention, the image size determining circuit 10 of Fig. 11 is modified to form an image size determining circuit 10' which generates a compression control signal CONT in addition to the signals HSET and VSET. Also, the signal processing circuit 5 of Fig. 11 is modified to from a signal processing circuit 5' which can perform data compression upon the video signal. That is, when a video signal of an image larger than an image having 1280 × 1024 dots is supplied to the signal processing circuit 5', data compression is performed upon the video signal in accordance with the compression control signal CONT including compression ratio information. In Fig. 16, which is a detailed circuit diagram of the image size determining circuit 10' of Fig. 15, the look-up memory 1005 of Fig. 12 is modified to from a look-up memory 1005' which generates the compression control signal COUT in addition to the signals HSET and VSET. The look-up memory 1005' is formed by a RAM or ROM for storing the values HSET, VSET and CONT as shown in Fig. 17. In Fig. 18, which is a detailed circuit diagram of the data processing circuit 5' of Fig. 15, this circuit 5' includes switching circuits 501 and 502, a data compression circuit 503, and a layer expansion circuit 504 which is used for distributing the video signal to the two signal line driving circuits 2-1 and 2-2. For example, if a first bit of the compression control signal CONT is "0", the switching circuits 501 and 502 are positioned on their upper sides as illustrated, so that data from the A/D converter 4 is supplied via the switching circuits 501 and 502 directly to the layer expansion circuit 504. Conversely, if the first bit of the compression control signal CONT is "1", the switching circuits 501 and 502 are positioned on their lower sides, so that data from the A/D converter 4 is supplied via the switching circuit 501 to the data compression circuit 503. Therefore, in the data compression circuit 503, data compression is performed upon the video signal using the compression ratio included in the compression control signal COUT in synchronization with the clock signal TCK. Then, the data compressed by the data compression circuit 503 is supplied via the switching circuit 502 to the layer expansion circuit 504. In Fig. 19, which is a detailed circuit diagram of the data compression circuit 503 of Fig. 18, this circuit 503 includes a frame memory 5031 for writing data from the switching circuit 501 and outputting data to the switching circuit 502. A write operation is carried out upon the frame memory 5031 by a write address counter 5032 whose write address WA is sequentially counted up by receiving the clock signal TCK. On the other hand, a read operation is carried out upon the frame memory 5031 by a read address counter 5033 and an address transforming look-up table 5034. In this case, a read address RA generated by the read address counter 5033 is sequentially counted up in synchronization with the clock signal TCK as shown in Figs. 20A and 20B. However, as shown in Fig. 20C, a read address RA' generated from the address transforming look-up table 5034 is shifted as compared with the read address RA in accordance with the compression ratio information included in the compression control signal CONT. Note that the compression ratio information is composed of a traverse ratio and/or a longitudinal ratio. Thus, even when a video signal of an image having more dots than 1280 in the traverse direction or more dots than 1024 in the longitudinal direction is input, the entire video signal is so compressed that it can be displayed on the LCD panel 1. In Fig. 21, which is a detailed circuit diagram of the layer expansion circuit 504 of Fig. 18, this circuit 504 includes D flip-flops 5041, 5042, 5043, 5044, and 5045, an inverter 5046, and a T flip-flop 5047. That is, the D flip-flops 5041, 5042 and 5043 are clocked by the clock signal DCK or its inverted signal having a period T as shown in Fig. 22A, while the D flip-flops 5044 and 5045 are clocked by a clock signal 2DCK having a period 2T as shown in Fig. 22B. Therefore, the outputs S1, S2 and S3 of the flip-flops 5041, 5042 and 5043 are obtained by delaying the output D of the switching circuit 502, as shown in Figs. 22C, 22D, 22E and 22F. On the other hand, since the flip-flops 5044 and 5045 are clocked by the clock signal 2DCK, the outputs DA and DB of the flip-flops 5044 and 5045 have the period 2T, so that the outputs DA and DB of the flip-flops 5044 and 5045 are as shown in Figs. 22G and 22H. In the above-mentioned embodiments, although the horizontal synchronization signal HSYNC and the vertical synchronization signal VSYNC are supplied independently of the color signals R, G and B, if the horizontal synchronization signal HSYNC and the vertical synchronization signal VSYNC are included in the color signal G, these synchronization signals can be derived from the color signal G. As explained hereinbefore, according to the present invention, even an image having a different size than an LCD panel can be displayed at a center portion of the LCD panel.

Claims: An apparatus for driving a liquid crystal display panel (1) having M signal lines (SG 0 , SG 1 ,···), N scan lines (SL 0 , SL 1 ,···), and M × N liquid crystal cells (C ij ) each connected to one of said signal lines and one of said scan lines, comprising: a signal line driving circuit (2-1, 2-2), connected to said signal lines, for sequentially driving said signal lines in response to a horizontal start timing signal (HS); a scan line driving circuit (3), connected to said scan lines, for sequentially driving said scan lines in response to a vertical start timing signal (VS); a horizontal timing generating circuit (8), connected to said signal line driving circuit, for receiving a horizontal synchronization signal (HSYNC) and delaying it with a first time period (HSET), to generate the horizontal start timing signal and transmit it to said signal line driving circuit; a vertical timing generating circuit (9), connected to said scan line driving circuit, for receiving a vertical synchronization signal (VSYNC) and delaying it with a second time period (VSET), to generate the vertical start timing signal and transmit it to said scan line driving circuit; and an image size determining circuit (10), connected to said horizontal timing circuit and said vertical timing circuit, for receiving the horizontal synchronization signal and the vertical synchronization signal, to calculate the first time period and the second time period in accordance with the size of the frame of a video signal and to transmit them to said horizontal timing generating circuit and said vertical timing generating circuit, respectively, said image size determining circuit comprising: means (1001, 1003) for calculating a horizontal frequency (f H ) in accordance with a horizontal synchronization signal (HSYNC); means (1002, 1004) for calculating a vertical frequency (f V ) in accordance with a vertial synchronization signal (VSYNC); means, connected to said horizontal frequency calculating means and said vertical frequency calculating means, for generating the horizontal start timing signal and the vertical start timing signal in accordance with the horizontal frequency and the vertical frequency, said horizontal frequency calculating means comprising: a frequency-to-voltage converter (1001) for receiving the horizontal synchronization signal; and an analog-to-digital converter (1003), connected to said frequency-to-voltage converter, said vertical frequency calculating means comprising: a frequency-to-voltage converter (1002) for receiving the vertical synchronization signal; and an analog-to-digital converter (1004) connected to said frequency-to-voltage converter, and wherein said horizontal start timing signal/vertical start timing signal generating means comprises a look-up table (1005). An apparatus as set forth in claim 1, wherein said image size determining circuit generates a compression control signal (CONT) including a compression ratio in accordance with the horizontal synchronization signal and the vertical synchronization signal, said apparatus further comprising: a data compression circuit (503) for receiving a video signal; switching means (501, 502), connected to said image size determining circuit and said data compression circuit, for supplying the video signal to said data compression circuit in accordance with the compression control signal. An apparatus as set forth in claim 2, wherein said data compression circuit comprises: a frame memory (5031) for storing the video signal; a write address counter (5032), connected to said frame memory, for generating a sequential write address (WA) and transmitting it to said frame memory, to thereby perform a write operation thereupon; a read address counter (5033) for generating a sequential read address (RA); an address transforming circuit (5034), connected to said read address counter, said frame memory, and said image size determining circuit, for transforming the sequential read address into a read address (RA') in accordance with the compression ratio included in the compression control signal and transmitting the read address to said frame memory, to thereby perform a read operation thereupon. An apparatus as set forth in claim 1, wherein said horizontal timing generating circuit comprises: a counter (801) for counting a clock signal (CK), said counter being cleared by the horizontal synchronization signal; and a coincidence circuit (802, 803), connected to said counter and said image size determining circuit, for determining whether or not the content of said counter coincides with the first time period, to thereby generate the horizontal start timing signal when the content of said counter coincides with the first time period. An apparatus as set forth in claim 1, wherein said vertical timing generating circuit comprises: a counter (901) for counting the horizontal synchronization signal, said counter being cleared by the vertical synchronization signal; and a coincidence circuit (902, 903), connected to said counter and said image size determining circuit, for determining whether or not the content of said counter coincides with the second time period, to thereby generate the vertical start timing signal when the content of said counter coincides with the second time period.

Inventors: SHIKI TATSUYA, SHIKI, TATSUYA, SHIKI, TATSUYA, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: G09G3
IPC codes: G09G3, G02F1, H04N3